CA1265275A - Dial pulse detection - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
DIAL PULSE DETECTION

A dial pulse detection system is disclosed which has an analogue input buffer, an analogue to digital converter (ADC), a processor and an output buffer. The processor operates on the digitally encoded samples produced by the ADC from the received signal to achieve detection of the dial pulse signalling data in the received signal. This detection process involves a training process and a recognition process. The training process extracts amplitude and timing characteristics of a first part of the received signal from a dialling instrument. These characteristics are then used to form a template for recognition of subsequent dialled digits.
Detected digits are then made available at the output buffer. The processor includes a facility for sending an error signal to the user to indicate that dialled digits have not been correctly received.

Description

?,7~ii SIGNALLlNG DE~ECTION

This invention relates to detection of signalling ~ata such as dial pulse signalling data.
In this specification and the appended claims, dial pulse signalling data means the make and break signals commonly produced by telephone dials, the pushbutton equivalent or the like and used in telecommunications systems for control and routing of calls through the telecommunications network.
In the field of telecommunications, systems for the o detection of dial pulse signalling are well known. These systems commonly employ a method of detection based on the observance of rigid amplitude thresholds and signal timing constraints. Such systems have taken many forms, from electromechanical devices using relays to detect the presence of dial pulses, to solid state electronics and microprocessor controlled systems which perform the same function somewhat more economically.
However, a problem arises in that the dc component of dial pulse signalling, used for setting up switch paths in the exchange, is removed at the first transmission bridge encountered in the path. From there on, only the ac component, generated as a consequence of the pulse shape of the dc signalling, is available for detection at various distant points in the network. Moreover, in order to use these ac components to control distant apparatus, they must be detected reliably with adequate discrimination against similarly generated interfering signals (which arise due to cross-talk on telecommunications lines) as well as any other background noise normally present in all electrical lal Jt~3~7~3 circuits.
An added problem, as far as reliabl~ detection of the aG component of the ~ignalling pul~es is concerned, ls that of timing. In the UK the normal speed of a tel~phone cl:Lal i~ 10 impul~e~ per secnnd (ips) but because of manufacturiny tolerances and wear, a range of 7-12 ips i5 speciEied. The variation in the speed of ~lectronically produced dial pulses (such as from a push button telephone) will of course be much less. The effects of amplitude distortion must also be considered, where the ampl1tude of ~ignalling pulses will vary according to the electr1cal characteri~tiç~ of the transmi6~ion medium and the distance over which dial pulses are to be detected. The transmission medium will al50 affect the timing of the signalling pulses.
Prior art systems, therefore, which use as the basi~ of their deteGtlon method, fixed amplitude and timing thresholds, cannot provide a reliable solution to the above problem~ particularly when attempting to detect pul~es at a cons~derable distance from the sencling end.
The pre~ent lnvention prov.icles a method and apparatu~ for detectlng dial pulse signalling data which can be both flexlble and rellahle in u~e.
According to a ~irst a~pect of the present invention there :1~ provided a method of detectlng dial pulse signalling data in a received ~ignal comprising the steps of: performlng a tralning process to train a dial pulse detector by processing ~ signal known to be a dialled signal to extract in:formatlon relating to the timing of the d.ialled pulses in a dialled digit in said dlalled signal, and per.forming a recognition proce~s by using said in~ormation to assist said detector in detectlng dial pul~e signalling data in the received signal.
AcGording to a second aspect of the present invention there is provided a detector for detecting dial pulse signalling data in a rece1ved signal comprising:
mean~ for processing a received signal including training 7~

means and reGognition means, said training means including means for proces~ing a signal known to be a dialled signal to extraGt information relating to the timing of the dialled pulses in a dialled digit in ~aid dialled ~ignal, and said recognitio~ means including means for operating on said information to assist in detecting dial pulse slgnalling data in the received signal.
According to a third aspect of the present invention there i~ provided interactive terminal equipment compris.ing a detector for detecting dialled digits comprising means for processing a received signal including training means and recognition means, said training means comprising means for processing a signal known to be a dialled signal to extract information relating to the tlming of the dialled pulses in a dialled digit in said dialled signal, said recognition means including means for using said information to assi~t in detecting dial pulse signalling data in dialled digits, and said means for processing inGluding means to transmit me~sages to a user inviting the user to dial digits according to the u~er's requ:lrements and to provide a service to the user dependent on the digits dialled.
A specifi~ embodiment of the inventlon will now be described, by way of example, with referenee to the accompanying draw.lngs, in which:
Figures l~a), l(b) and l(c) are waveform diagram~ showing the eleGtrical wave~orm of idealized dial pulse ~ignals:
(a) before a first transmission bridge;
(b) after a first transmission bridge; and (c) at a distance receiving end, respectively;
Figure 2 is a schematic block diagram of a system for implementing the method of detection of dial pulse signalling data acGording to the invention;
Figure 3 is a flowGhart of the dial pulse training process performed by the system of Figure 2;
Figure 4 shows the format of deteGtion windows - 3~ -set up by the ~ystem of Figure 2 and used for performing the di~l pul~e recognition process; and 3_ Figure 5 shows an example of a pulse array set up by the system of Figure 2 as a result of the dial pulse recognition process.
Figures l(a), (b) and (c) show idealized verslons of the electrical waveforms of analogue dial pulse signals. Figure l(a) shows a portion of dlal pulse signalling in its relatively distortion free state before entering a dc block (ie before reaching the first relay set of transmission bridge in the local exchange). Time interval t1 represents a make period and interval t2 represents a break period. The make/break period t3 is therefore made up of the sum of the two periods t1 + t2.
Make period t1 i5 nominally 331/3 ms and break period t2 nominally 662/3 ms, thus givin~ rise to a make/break period t3 of 100 ms corresponding to a mean dial pulse signalling rate of 10 impulses per second (ips). There will be a tolerance in the values of intervals t1 and t2 since dial signalling rates may be allowed to vary from 7 to 12 ip5 thus altering the make/break pulse period t3.
~ny ~iven dial will produce relatively constant values for period3 t1 and t2 but this will vary from dial to dial and from one line to another.
Figure ltb) shows the effect of passing the signals of Figure l(a) through a first transmission bridge ~(a dc block). Figure l(c) shows idealised ringing distortion ~ue to passing the signal of Figure l(b) along a transmission medium. It should be noted that the waveforms of E'igures l(a) - l(c) and in particular Figure l(c) will normally be sub~ect to additional random distortion effects (not shown) of background noise and impulsive interference.
~ eferring to Figure 2, a dial pulse detection system 20 is shown in which analogue dial pulses of the type shown in Figures l(a) or l(b) or l(c) are input at 21 to an input interface 22, from, for example, exchange equipment 19. The analogue dial pulses are then passed to a detector 23. The detector 23 comprises an analogue to digital converter 24 and a microprocessor system 25. The converter 24 converts the analogue dial pulses into digital samples which can be read by the microprocessor 25. Processed output from detector 23 passes via interface 26 and is output at 27. Input and output interfaces (22, 26) ensure that all signal levels are compatible and provide protection for detector 23. Input interface 22 also includes a low pass filter ~not shown) in order to remove unwanted higher frequency components in the received signal and thereby aid detection.
In operation, the dial pulse detector of Fi~ure

2 would normally be located at a distance receivin~ end on a telecommunications line. A user would therefore be able to route his call through the exchange e~uipment 19 (having dialled the required number in the normal fashion) and would then be able to supply extra dialled digits over the establishecl link to the detector. The detector may therefore be used to detect this dialled signalling information which may in turn be used for control of other equipment to provide a required service to the user. For example dialled di~its may be used to control computerised data bases; for implementin~ automatic operator facilities for PABXs; and in interactive answerin~ machine systems.
A typical automatic operator application of the invention in a PABX ~ystem would enable an outside exchange line user to dial any extension on the PABX. The detection 6ystem 20 would, for example, be located in or before the PABX. A telephone call for the PABX would be intercepted by the detection system which would then play a messa~e to the caller. The detection system would detect additional digits dialled by the user, i.e. the extension number, and pass the detected number on to the PABX equipment to route the call through to the required extension. In a preferred embodiment of the invention, this would be achieved by the user dialling the extension number preceded by a training digit with a long pulse train (e.g. a nine).
The invention may also be easily incorporated in systems requiring interactive detection of dial pulse signalling data and one example of such a system would be o an intelligent controller for accessing a computerised data base. The intelligent controller would send instructions over the exchange line to the user (e.g. to access file X
dial 123) who would then respond by dialling the appropriate digits. The dial pulse detection system 2û
would then pass the detected digits to the controller in order to initiate the appropriate action. In such a system, security of access could be achieved by ensuring that each user dials a unique number for access to the data base With the detector located at a distant receiving end on a telecommunications line, the type of dial pulse signalling available to the detector will be of the form shown in Figure l(c). Here, the regular loop/disconnect (make/break) pulses of Figure l(a) have been degraded by 2s the transmission bridge and the characteristics of the line into a series of positive and negative going oscillations which decay in time. These oscillations are excited by the transients of Figure l(b) which correspond to the rising and falling edges of the original loop/disconnect signal of Figure l(a~. Additionally, the signal of Figure l(c) will also contain noise (not shown) which may take the form of both random impulsive interference (mostly due to crosstalk ;c~7~

from other lines carrying dial pulses) and general background noise. At a distant receiving end, the amplitude of the signal of Figure 1 (c) typically varies greatly with a mean peak level of approximately 2 volts and a standard deviation of approximately 1.5 volts.
Background noise typically ranges from 50 mV - 100 mV with impulsive noise often reaching much larger levels The signal of Figure l(c), along with associated noise as described above, is therefore input to the detector of o Figure 2 at input interface 22~ Input buffer 22 is designed to cope with the above variation in input signal level. Input buffer 22 incorporates a low pass filter in order to reject the unwanted higher frequency components (primarily above 4kHz) present in the received signal~
This low pass filter band limits the received signal to 4kHz thereby retaining the main part of the signalling information and enabling the subsequent use of an 8 kHz sampling frequency. Additionally, since most of the signalling information is conveyed at frequencies below 2 khZ, removal o~ higher frequencies by a second low pass filter serves to improve the subsequent detection process (described below). lhese filtering operations could be combined into one operation. Buffer 22 incorporates gain control circuitry which serves to provide an optimum signal level for operation of the detector 23 and also protects the detector 23 from damage arising from any excessively large voltage translents at the input 21. Buffer 22 also provides dc isolation of the detector 23 from the telephone line at input 21.
The analogue signal then passes to the analogue to digital converter (ADC) 24. ADC 24 is a conventional device which produces digital codes compatible with the J

I

input of processor 25 In the present implementation standard A-law encoding is used. ~owever, since maximum negative going peaks in the analogue signal (due to the falling edge of the long break pulse) tend to be larger than any positive going peaks, a digital encoding technique is used which full wave rectifies the received signal by taking magnitude of the received signal as a positive modulus. All analogue voltages are therefore encoded as digital samples having a o positive modulus. The samples produced by ADC 24 are then made available for processing by the microprocessor system 25. Processor 25 operates on the samples in two distinct modes to achieve detection of the dial pulse signalling data. The first mode is a training process, and the second lS mode is a recognition process.
The training and recognition processes performed by processor 25 are described in detail below with reference to Figures 3 to 5.
Figure 3 is a flow chart of the operation of the training process. The purpose of this training process is to obtain a "signature" or template of various signal parameters relating to the dialling instrument to which the detector is to be trained. For this reason, the user would (after a connection to the detector has been established) dial a predetermined digit intended solely for the purpose of training the detector to that dial and line. In this example, the training digit is a nine which gives rise to a train of 9 make/break pulses, giving the detector a good opportunity to train to the signal parameters of the dial and the line. In addition to these signalling pulses, telephone instruments produce extra pulses at the beginning and end of dialling. These are termed "off-normal" pulses and are caused by the s~itching between voice and dial signalling circuitry within the telephone, and occur as single pulses before and after the make/break pulse sequence.
Figure 3 shows this training process, in which a template of the parameters of the dial and line is formed using measured signal levels and times. On receiving a stream of training pulses, the processor 25 analyses the digital samples from ADC 24 which represent the received o signal. Processor 25 looks for those samples representing a first maximum peak whereupon a detection window is held open for 9 ms. All samples received while this window is open are analysed and a maximum peak level occuring within this period is recorded. The window is then closed, and there is a delay of 19 ms before any further samples are taken. Following this, another window is opened on detection of the next peak and a maximum peak level obtained within this period is again recorded. Processor 25 calculates the time between these maximum peaks and this parameter is stored. If this time is found to be less than 53 ms, then this is taken to indicate that the interval between the previous two peaks constituted a make period (i.e. t1 ideally 331/3 ms as in Figure 1). There is then a delay of 50 ms before beginning the next analysis in order to await the arrival of the peak representing the end of the subsequent break period (and the beginning of the next make period). Should that time period between the first two peaks be found to be greater than 53 ms, thus indicating the occurrence of a break period (i.e. t2 ideally 662/3 as in Figure 1), then a delay of 19 ms is used before beginning the next analysis which would expect the arrival of a pulse representing the end of a subsequent make periocl (and the beginning of the next break period).
Analysis finishes when the parameters of three pulses (representing a complete make/break period t3 as in Figure 1) have been recorded. If the total time between first and last pulses is found to be outside a specified tolerance (where this tolerance corresponds to the allowed 7 ~12 ips rate, i.e. a range of 83 ms to 143 ms) then all data is erased and training is restarted. If not, a template is prepared using the measured break period, make o period, maximum peak of break pulse, and maximum peak of make pulse. The periods are measured between the maximum peaks.
The fact that the training process is restarted if the above tolerance is exceeded means that errors which might otherwise arise due to noise pulses in the received signal are eliminated. The system therefore ensures that any pulses due to noise will be ignored since such pulses are essentially of a random nature and will have a different level to those of the dial pulses. Large amounts of noise on the receivecl signal may cause the operation of several training cycles before the dial parameters are successfully obtained. The use of, say, the digit nine for training purposes, allows several training cycles. If for some reason (perhaps due to excessive noise in the received signal, or the use of a faulty dial) detector 23 finds it impossible to train to the dialling instrument, then processor 25 will generate an error signal (which could be in the form of a tone or a standard voice synthesised message) to indicate to the user that there has been a 30 malfunction and that the user should re~dial.

7~

~ormally, the user will dial a series of digits, the first of which will be a training digit (for example the digit nine), and subsequent digits are then detected by processor 25 using the parameters obtained from the training digit in the course of the training process. In order to do this, processor 25 begins the recognition process once the training process has been successfully completed. The operation of the recognition process is shown in Figures 4 and 5.
o The recognition process uses the dial pulse characteristics obtained in the training process to set up a series of analysis windows. Any samples received outside these windows are ignored. The advantage of setting up these analysis windows according to the dial pulse information from the training process is that the width of each window can be made small (9 ms in this example) in comparison to the dial pulse period (an average of lûO ms). In this way only small amounts of signal need be analysed at any one time in order to be certain of receiving valid dial pulses in the presence of impulsive noise. Figure 4 illustrates the time format 40 of the analysis windows 42 observed by processor 25 after the first mal<e pulse 41 has been received (corresponding to the first pulse of the digits which are to be detected after the training digit). Each analysis window 42 is associated with a location 51 in the pulse-array 5û (of Figure 5) which is built up progressively and stored by processor 25 to describe the incoming pulse train. The analysis windows 42 are numbered from û to 24 where the first window is numbered 0. For clarity, Figure 4 shows an expanded view of windows 8, 9 and lû. This numbering of the windows 42 corresponds to the numbered locations 51 in the array 50 of Figure 5. Each location 51 of array 50 stores the result of its correspondingly numbered analysis windows 42. Pulse array 50 corresponds to an example recognition of the dialled digit six.
The analysis windows 42 are opened at regular intervals corresponding to the measured make and break periods (t and t2 respectively) from the training process. While each analysis window 42 is open, and this is only for a period of 9 ms9 processor 25 compares the amplitude of o received samples against the appropriate make or break peak value expected for that particular dial (from the training process). If the amplitude of a sample is found to fall within an acceptable tolerance of the expected peak value, then a valid pulse is determined to have been received, and the correspondingly numbered location in the pulse array 50 of Figure 5 is flagged true. If an analysis window 42 should time out with no valid pulse being received, then the corresponding element of the pulse array 50 is flagged false. Array 50 o~ Figure 5 shows each location 51 flagged in this way, either with a T (i.e. true), or an F (i.e.
false).
As soon as either of these conditions occur, processor 25 causes a delay until the next analysis w.indow 42 is due to be opened, where this delay is determined by the appropriate measured tl or t~ pulse period from the training process. All even numbered locations (0,2,4,6 ...) in array 50 will hold a true or false result representing a valid or invalid make pulse received (this being so since it is assumed that the first pulse received must be a make pulse). All odd numbered locations (1,3,5,7,...) in array 50 will hold a true or false result representing a valid or invalid break pulse received.

7~;

Locations (0,1) therefore represent the first dial pulse digit received (i.e. one make and one break), ~ith locations (2,3) representing the second dial pulse received, and so on.
s Processor 25 causes the analysis windows 42 to be opened and closed according to the format 40 of Figure 4, until any of the following conditions are determined to be true:

o 1) Any three pulses have been missed, and the number of pulses correctly received is less than four. This condition is used to detect quickly when a noise spike has been received instead of the first make pulse, enabling the processor 25 to reset the system before the correct pulse appears. This mechanism also allo~.~s an "off normal" pulse (which occurs one Inter-Digit Pause (IDP) period before the dialling pulses) to be discarded.

2) Four consecutive pulses have been missed.
Processor 25 interprets this as the Inter Digit Pause which represents the minimum pause between dialled digits and has a value of at least 330 ms.

2s 3) Any five pulses have been missed. Probably due to an IDP with a noise spike triggering a single valid pulse condition.

4) More than 22 analysis windows have been opened.
Although this is unlikely, it is possible that noise spikes would prolong the pulse train, thereby giving an erroneous result if detection ended abruptly after 20 S pulses (i.e 10 makes and 10 breaks which represents the dialled digit zero). This small additional margin acts as an aid in the detection of errors (see below).

To implement the detection of conditions (1) to (3), o processor 25 operates a missed pulse counter which is incremented on every failure to recognise a valid pulse in each analysis window. Processor 25 resets this missed pulse counter on each occurrence of two consecutive correctly received pulses, under the assumption that noise spikes, being essentially random in nature, are unlikely to cause this effect and processor 25 therefore determines that, since the pulse train is still being received, all previous missed pulses were mistakes. Errors will also be detected which arise from any pulses being received in windows 20 to 24 (area 43 in Figure 4) which cause any of locations 20 to 24 to be flagged true. Processor 25 therefore determines that such locations flagged true must be due to noise since no valid digits greater than the digit zero can be received.
When the above procedure is complete and the pulse array has been compiled, processor 25 determines the value of the received digit by searching backward through the . pulse array for the first occurrence of two consecutive correctly received pulses which correspond to a valid digit received. It then calculates the number dialled from the position in the array that this occurs. Figure 5, 2~

therefore, shows one example of the state of the pulse array 50 after reception of the dialled digit 6 where locations (10, 11) hold the last consecutively valid entries which correspond to the dialled digit 6. In this example, 52 of Figure 5 shows location 3 which is the first to be flagged false. The missed pulse counter operated by processor 25 therefore records one missed pulse. However 53 shows the digit 3 which is subsequently correctly received (locations 4 and 5 flagged true) and the missed o pulse count is therefore reset to 0 (as described above).
Similarly, missed pulses in locations 6 and 8 increment the missed pulse count to two but this is then reset at 54 by the valid pulses in locations 10 and 11. Locations 12, 13, 15, 16 and 17 are then flagged false, with only a single valid pulse being indicated at location 14, which indicates that a fifth pulse has been missed without correction.
This condition causes processor 25 not to open any more analysis windows (i.e. window 18 is not opened) and the processor determines that all pulses in the dialled digit have been received and that an Inter Diyit Pause has just been received (from condition 2 above). Processor 25 then searches the array 50 by working back from 55 (location 17 - the last one flagged) to find the first instance of consecutive valid pulses (i.e. flagged true). In this example this first occurs at 54 (locations 10 and 11).
Since locations 10 and 11 correspond to the dialled digit six, processor 25 determines that the dialled digit was in fact a six and no further locations are searched. In this example, if location 12 was also true, and 1~ false, then the dialled number would still be recognised as a six (with the location 13 being ignored as a single error pulse).
This condition will also prevent an "off-normal" pulse (which occurs after the dialling pulses) being treated as a dial pulse. This "off-normal" pulse occurs at approximately one break period after the last make dial pulse transient and thus could be recognised as a valid 5 break pulse transient. If location 12 was false and locations 13 and 14 true then the digit received would still be recognised as a six since the rule of two consecutive true locations implying a valid pulse period can only apply in locations corresponding to a digit value o (i.e O and 1, 2 and 3, 4 and 5etc.) and not in locations falling between digit values (i.e 1 and 2, 3 and 4, etc).
The recognition process therefore employs error detection and correction embodied in the missed pulse counter and its reset by processor 25 according to the above conditions.
If, for some reason, it is not possible for processor 25 to perform a satisfactory recognition process (as in, for example, condition 4 above) then the processor will cause a malfunction signal to be sent to the user in the form of a tone or a voice synthes.ised message in order that the user will understand that the number is to be redialled.
Once the recognition process has been performed successfully on a dialled digit, processor 25 passes its numerical value to output buffer 26 (shown in Figure 2) where it is made available at the output 27 for use by other equipment as described above. Processor 25 is then ready to repeat the recognition process on any subsequent digits dialled by the user. When all digits have been successfully received, processor 25 will also cause a voice synthesised message to be sent to the user to confirm the value of the dialled digits received and stored in buffer 26.

In the specific embodiment described above, the training process is carried out on a predetermined training digit which is used solely for training purposes and is not intended to be recognised by the system. Alternatively, 5 the training process could be "implicit" and form part of the recognition process thus obviating the need for a separate training digit. In this case, the first digit would be used to train the detector to the dialling instrument, but would at the same time be itself recognised o by the detector as ~ell as being used as a basis for recognition of subsequent digits. This implicit training would require the first attempt at recognition to have the widest tolerances on pulse parameters which would have to be according to an average or optimum setting based on experience. Subse4uent dialled digits would, however, be recognised according to a template prepared from the measured parameters of the first digit received.
If desired, further parameters may be extracted from the dial pulses during training to assist in detection.
One such parameter is the polarity of the first peak in the transient hhich indicates whether a transient was generated by a make or break pulse. Also, the number of zero crossings in the ringing of the transient (i.e. the number of oscillations) could be used to improve immunity against noise.
In the traininy process, it may be advantageous to provide default values for the dial pulse parameters, so that some or all of the default values can be used if the training is incomplete or unsuccessful.

7~:9 Another option is to analyse the received signal between the detection windows to compare the level of the signal with the level of the make or bre3k pulses measured during training. The detector can be made to ignore continuous signals such as speech which coul~ otherwise cause false recognition.
Although a dial pulse detection system has been described which performs analogue to digital conversion of the received analogue signal in order to process the signal o in digital form, it is of course operable without such conversion in situations (such as in a wholly digital network) where the detector can be directly presented with the signal in digital form.
Additionally, further measures can be taken to improve 15 the performance of the detector 25 when dealing with received signals known to have a very poor signal to noise ratio. Such measures may include the use of additional signal pre-processing at the input buffer stage 22. This pre-processing could take the form of analogue or digital filters to enhance the signal to noise ratio of the received signal and thus provide for optimum operation of the detector.

Claims (32)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of detecting dial pulse signalling data in a received signal comprising the steps of:
performing a training process to train a dial pulse detector by processing a signal known to be a dialled signal to extract information relating to the timing of the dialled pulses in a dialled digit in said dialled signal; and performing a recognition process by using said information to assist said detector in detecting dial pulse signalling data in the received signal.

2. A method of detecting dial pulse signalling data as claimed in claim 1, wherein said information includes measured values for a maximum make pulse level, a maximum break pulse level, a make period, a break period, and a total dial pulse period.

3. A method of detecting dial pulse signalling data as claimed in Claim 2, wherein the training process is re-started if a value obtained for the total dial pulse period is outside of a given tolerance.

4. A method of detecting dial pulse signalling as claimed in claim 2, wherein the recognition process comprises measuring the maximum make pulse level of received signals during time periods spaced according to the measured spacing of pulses during the training process.

5. A method of detecting dial pulse signalling data as Claimed in Claim 4, wherein said time periods are less than 15 milliseconds.

6. A method of detecting dial pulse signalling data as claimed in claim 4 or Claim 5, wherein the recognition process includes comparing the level of received signals during said time periods with the pulse levels derived from the training process to detect the presence or absence of valid dial pulses.

7. A method of detecting dial pulse signalling data as claimed in claim 6, wherein the recognition process includes storing information upon the detection of the presence or absence of valid dial pulses for each time period in respective locations of a data store.

8. A method of detecting dial pulse signalling data as claimed in claim 4, wherein the recognition process is terminated and/or re-started according to a count of pulse absences.

9. A method of detecting dial pulse signalling data as claimed in claim 8, wherein said count of pulse absences is reset by subsequent detection of the presence of at least two consecutively valid dial pulses.

10. A method of detecting dial pulse signalling data as claimed in claim 7, wherein said recognition process derives a detected value of a dialled digit by examination of the information stored in the locations of the data store representing at least two consecutively valid dial pulses.

11. A method of detecting dial pulse signalling data as claimed in claim 1, 2 or 3, wherein if the training process fails to extract said characteristics from said portion of the received signal, the training process will transmit a failure message.

12. A method of detecting dial pulse signalling data as claimed in claim 1, 2 or 3, wherein said recognition process includes transmitting a message to indicate the detected dial pulse signalling data.

13. A method of detecting dial pulse signalling data as claimed in claim 1, wherein the received signal comprises digitally encoded representations of telephone dial pulses.

14. A method of detecting dial pulse signalling data as claimed in claim 13, wherein the portion of the received signal processed by the training process comprises representations of at least four dial pulses.

15. A method of detecting dial pulse signalling data as claimed in claim 1, wherein said training process includes measuring time intervals between successive peak signals and storing time information relating thereto.

16. A method of detection dial pulse signalling data as claimed in claim 1, wherein said training process includes measuring an amplitude relating to the peak of the signals and storing amplitude information relating thereto.

17. A method of detecting pulse signalling data as claimed in claim 15, wherein the training process includes detecting the presence of erroneous signalling information from the stored time information, erasing the stored time information, and restarting the training process using further received signals.

18. A method of detecting dial pulse signalling data as claimed in claim 15, including providing a series of analysis windows computed from the stored information for use in the recognition process.

19. A method of detecting dial pulse signalling data as claimed in claim 18, including determining during each analysis window whether a signal has been received corresponding to that anticipated as a result of the training process and storing information thereon.

20. A detector for detecting dial pulse signalling data in a received signal comprising: means for processing a received signal including training means and recognition means, said training means including means for processing a signal known to be a dialled signal to extract information relating to the timing of the dialled pulses in a dialled digit in said dialled signal, and said recognition means including means for operating on said information to assist in detecting dial pulse signalling data in the received signal.

21. A detector for detecting dial pulse signalling data as claimed in claim 20, wherein said processing means includes means for measuring a maximum make pulse level, a maximum break pulse level, a make period, a break period, and/or a total dial pulse period.

22. A detector for detecting dial pulse signalling as claimed in claim 21, wherein the measuring means is configured to measure the maximum make pulse level of received signals during time periods spaced according to the measured spacing of pulses during the training process.

24. A detector for detecting dial pulse signalling data as claimed in claim 22, wherein the recognition means includes means for comparing the level of received signals during said time periods with the pulse levels derived from the training means to detect the presence or absence of valid dial pulses.

24. A detector for detecting dial pulse signalling data as claimed in claim 23, wherein the recognition means includes storage means for storing information upon the detection of the presence or absence of valid dial pulses for each time period.

25. A detector for detecting dial pulse signalling data as claimed in claim 20, including means for transmitting a failure message if the training process fails to extract said characteristics from said portion of the received signal.

26. A detector for detecting dial pulse signalling data as claimed in claim 20, wherein the training means includes means for measuring time intervals between successive peak signals and means for storing time information relating thereto.

27. A detector for detecting dial pulse signalling data as claimed in claim 26, including measuring means for measuring an amplitude relating to the peak of the signals and means for storing amplitude information relating thereto.

28. A detector for detecting dial pulse signalling data as claimed in claim 26, wherein the training processor includes means for detecting the presence of erroneous signalling information from the stored time information, means for erasing the stored time information, and means for restarting the training process using further received signals.

29. A detector for detecting dial pulse signalling data as claimed in claim 26, including means for providing a series of analysis windows computed from the stored information for use by the recognition processor.

30. A detector for detecting dial pulse signalling data as claimed in claim 29, including means for determining during each analysis window whether a signal has been received corresponding to that anticipated as a result of the training process and means for storing time information thereon.

31. Interactive terminal equipment comprising a detector for detecting dialled digits comprising means for processing a received signal including training means and recognition means, said training means comprising means for processing a signal known to be a dialled signal to extract information relating to the timing of the dialled pulses in a dialled digit in said dialled signal, said recognition means including means for using said information to assist in detecting dial pulse signalling data in dialled digits, and said means for processing including means to transmit messages to a user inviting the user to dial digits according to the user's requirements and to provide a service to the user dependent on the digits dialled.

32. Interactive terminal equipment as claimed in claim 31, wherein the equipment is associated with an exchange to provide an automatic operator service so that an outside exchange line user is able to dial any extension on the exchange.